The task of counting migrating fish is one that has been dealt with over the past many years either by direct visual counting or through the use of various counting devices. In U.S. Pat. No. 4,034,331 patented by Albert S. Menin and Robert M. Bridges (common assignee), a sonar fish counter is disclosed which includes a straight, relatively stiff pipe or ladder-like structure forming an artificial bottom and from which is suspended a webbing or similar type of obstacle which makes contact with the actual river bottom to force the fish to swim over the artificial bottom structure. A side scanning sonar transducer is then used to insonify the volume of water immediately over the artificial bottom, receiving echoes from the fish swimming over this structure. This type of system is quite satisfactory for counting salmon migrating in streams, as they do in Alaska and many other streams flowing into the Pacific Ocean, since they tend to swim in relatively well-defined channels close to the stream bottom. This type of counter cannot accurately count fish of other varieties or salmon which do not tend to swim in closely defined channels since the beam width is such that the sonar would inherently miss counting a high percentage of the fish. It has become apparent that a substantially different type of counter is required for counting fish in large rivers and estuaries where large areas must be covered by the sonar.